Typical sound processing devices, such as hearing aids, comprise a microphone or other input transducer to pick up acoustic sounds and convert them into an electrical signal, an electronic processor and/or amplifier to increase the power of the electrical signal, and a speaker or other output transducer to convert the amplified electrical signal back into acoustic sound. If the input and output transducers are close enough, the output acoustic signal may be picked up by the input transducer and fed back into the amplifier with a delay, the delay being the time taken for the sound to travel from the output transducer to the input transducer, plus any delay due to the electrical processing of the signal. This is ‘acoustic feedback’. Electrical feedback can also occur if the electrical signal at the output is coupled back to the input, for example by inductive or capacitive coupling. Further, mechanical feedback can also occur if vibrations are transmitted from the output transducer to the input transducer via the body or case of the amplification system.
Under feedback conditions the loop gain is greater than 1, such that the feedback signal self-reinforces and increases in intensity to drive the components into saturation, reaching an equilibrium when the loop gain reduces to unity. At this equilibrium level the hearing aid device usually emits a continuous and unpleasant high pitched whistle or squeal. Further, oscillation and instability in the processing path are undesirable because they can distort the signal processing performance. This can lead to problems both for the hearing aid user and for those around.
One approach for increasing the stability of a hearing aid is to reduce the gain at high frequencies. In multi-band processing this may be done by setting a maximum gain value for each band which reduces with increasing frequency, or automatic high frequency (HF) gain roll-off may be used. However, this means that the desired high-frequency response of the instrument must be sacrificed in order to maintain stability, which is particularly undesirable given that human hearing loss often occurs to a greater extent in the higher audible frequencies than in the lower frequencies.
Efforts have also been undertaken to reduce the susceptibility of hearing aids to feedback oscillation by attenuation and notch filtering; estimation and subtraction of the feedback signal (feedback cancellation); and frequency shifting or delaying the signal.
A further difficulty in feedback cancellation arises where an input sound signal comprises tonal and other periodic signals which ideally should not be cancelled, such as music, beeps, dial tones and the like. Such tonal signals can be difficult for signal processing techniques to distinguish from oscillatory feedback which should be cancelled. For example, some feedback cancellation techniques assess an auto-correlation of an input signal, and attempt to filter out signals with a high correlation, oscillatory feedback being one such signal with high correlation. However, tonal signals of interest such as music also have a strong auto-correlation, with the result that feedback cancellation is inappropriately applied to the music signal in such techniques. This can result in a decreased efficacy of cancellation of actual feedback signals occurring simultaneously with the tonal input, and/or the production of audible artefacts such as ‘warbling’ when the tonal signal of interest is present. Such artefacts can also arise if adaptive feedback cancellation techniques cause a feedback estimation filter response to alter at a rate or by such an amount as to be perceptible to the user.
To provide a feedback estimation filter which responds appropriately to both tonal input signals and oscillatory feedback signals, respectively, some solutions utilise training to set a fixed filter response. However, such filter training necessitates an extra step in hearing aid fitting or implementation. Further, such fixed filters tend to have a limited range of situations in which feedback cancellation is adequately provided.
Other solutions utilise complicated tone detectors to detect situations where signals are present which include tones which could cause artefacts, for example by corrupting the filter taps. However, not all tones lead to corruption of filter taps, and such systems can thus detect a tone and reach a false positive determination that a filter has been corrupted, even when the filter has not been corrupted or has not been unacceptably corrupted. Conversely, tonal signals which may not be detected by a tone detector can nevertheless cause filter corruption, leading to a false negative determination. Some systems use a tone detector in order to control the adaptation rate of the feedback cancellation filter. By slowing the adaptation rate of the filter the presence of a tonal signal is less likely to corrupt the filter and give rise to artefacts. However a slowly adapting filter is susceptible to producing short bursts of feedback squeal if the feedback path changes faster than the FBC's adaptation rate.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
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